1. Field of the Invention
This invention relates generally to printing apparatus and, more particularly, is directed to printing apparatus of the thermal transfer type.
2. Description of the Prior Art
Apparatus for printing visual information on recording paper in response to an information signal are well-known in the art. One such printing apparatus is of the thermal transfer type in which a pigment is selectively transferred from a tape to a record medium, such as a sheet of paper, by applying thermal energy to localized areas on the tape. As an example, a thermal head assembly of the apparatus may include a plurality of thermally excitable elements which, when activated, transfer the pigment to the paper as an arrangement of dots or other discrete elements.
Generally, the tape used with such printing apparatus is wound about two reels which are then positioned in the apparatus, with the segment of tape extending between the reels being positioned between the thermal head assembly and paper. With such arrangement, the thermal head assembly is moved in the longitudinal direction of the platen in contact with the segment of tape during the recording operation to transfer the pigment from the tape to the sheet of paper. After a horizontal line has been printed on the sheet of paper, the thermal head assembly is moved away from the platen out of contact with the tape and returned to its original position to begin printing another horizontal line on the sheet of paper.
During the return operation, a mechanism associated with the thermal head assembly is used for clamping the tape to withdraw an unused portion of tape from the supply reel along therewith for use in the printing operation of the next horizontal line. Such clamping mechanism may take the form of a pair of pinch rollers which pinch the tape therebetween. Such arrangement, however, may be unstable in that the tape held between the pinch rollers may slip during movement thereof in the return operation. In such case, a portion of the tape to be used for recording the next horizontal line may include both used and unused portions of tape.
Further, during the recording operation, that is, during the recording of each horizontal line on the paper, the segment of tape extending between the supply and take-up reels should be fixed so that the thermal head assembly slides therealong to always record on an unused portion of tape. However, during the recording operation, as a result of the heat generated by the thermal head assembly, the tape often sticks to the thermal head assembly. In such case, a used portion of tape will be transported with the thermal head assembly and repeatedly used during the printing of a horizontal line, with consequent deterioration of the printed line.
Another problem that may occur during the recording operation is that elongation of the tape extending between the thermal head assembly and the supply reel may be caused as a result of the heating of the tape by the thermal head assembly and friction existing between the tape and thermal head assembly. If such elongation is not corrected, sag or slack in the tape caused by such elongation may result in misalignment of the tape on a guide roller or pin positioned between the thermal head assembly and supply reel. In such case, the tape may be guided or ride on the rim or flanged portion of the guide roller or pin, resulting in a consequent misalignment of the tape and thermal head assembly. In such case, normal contact of the thermal head assembly on the tape during the recording operation may not be effected so as to result in a deterioration of the printed line of information.